In general, when extruding a billet comprised of a metal material, for example, aluminum or its alloyed material, etc., by an extrusion press machine a hydraulic cylinder drives a ram. At the front end part of the ram, a stem is attached. A container is pushed against the die. In that state, the billet is inserted into the container by the stem, etc. The ram is further advanced by operation of the hydraulic cylinder whereby the billet is pushed by the stem. Then, a shaped product is extruded from the exit part of the die.
After the billet, which is made smaller in diameter than the inside diameter of the container, is inserted in the container, if the billet is pushed inside the container against the die by the stem at the back of the billet for so-called upset, the billet is crushed, whereby the center swells resulting in a barrel shape. Between the container and the billet, in particular at the die side end face part of the inside wall of the container, air is compressed and builds up. To discharge the compressed air which has built up inside the container, the stem and the container are slightly retracted, whereby a clearance is provided between the die end face and the container end face and the compressed air is discharged, then the container and stem are again advanced to start the extrusion. The process of discharging the compressed air in the cylinder in this way is referred to as “burp cycle”.
In a conventional extrusion press machine, a prefill valve is provided as a means for supplying and discharging a hydraulic oil in the main cylinder in a short time at the time of making the ram move in the main cylinder with no load. At the time of movement of the ram at the time of no extrusion, the prefill valve is unavoidably operated. Time is required at all times for operating the prefill valve from when moving by a full stroke. For this reason, if performing the burp cycle in the extrusion process, the cycle time becomes longer.
To solve this problem, a plurality of hydraulic working valves which communicate with the main cylinder and drain the hydraulic oil even in a state closing the prefill valve when refracting container cylinders and retracting the ram and are independent from the prefill valve are provided at the back part of the main cylinder in this configuration. Due to this configuration, it is possible to supply and discharge hydraulic oil in the main cylinder without operating the prefill valve at the time of the retraction operation of the ram in the burp cycle and it is possible to shorten the burp cycle time (see PLT 1).
In this regard, as shown in FIG. 3, in the conventional type of extrusion press machine which is disclosed in PLT 1, at the time of upset, the hydraulic oil which is supplied to the main cylinder is lowered in pressure by the hydraulic valve which is provided for lowering the pressure, then a hydraulic valve which is provided for the burp cycle is opened to discharge the hydraulic oil in the main cylinder to a tank during a container- and ram-retraction operation of the burp cycle and hydraulic oil is supplied from a hydraulic pump to the container cylinders to retract the ram through the container and extrusion stem. Furthermore, the hydraulic valve for burp cycle use which is opened after the completion of retraction of the container is closed, then the container and ram are advanced again. For this reason, time is required for the processes from after the completion of upset to readvance of the container and ram and time is wasted for the burp cycle process.